This invention relates to a control unit for use in an injection molding machine which is driven by a servo motor and which will be referred to as an electrical injection molding machine hereinunder.
Recently, an electrical injection molding machine of the type described, which is driven by a servo motor in place of a hydraulic actuator, has widely been used to inject a resin material into a mold and to manufacture a molded product. In general, the injection molding machine includes an injection unit driven by the servo motor and a controller coupled to the injection unit and the servo motor to control the servo motor by monitoring the injection unit. The injection unit comprises an injection cylinder provided with a hopper and a screw arranged in the injection cylinder. The screw is directly coupled at its rear end to a drive shaft operatively coupled to an injection motor formed by the servo motor. The servo motor rotates the drive shaft to move the drive shaft forwards and backwards. As a result, the screw reciprocates forwards and backwards along a guide bar. A load cell is mechanically coupled to the drive shaft to detect a pressure imposed on the drive shaft.
Description will hereinafter be given as regards an operation of the injection unit.
(1) A resin material is introduced from the hopper into the injection cylinder and is melted into molten resin. The molten resin is conveyed towards a top end portion of the heat cylinder by a predetermined amount by driving the injection motor. This process is referred to as a measuring process. During the measuring process, the screw is slightly retracted due to a back pressure of the molten resin packed into the top end portion of the injection cylinder. The back pressure is detected by the load cell which produces a pressure detection signal representative of the back pressure. The back pressure is controlled by the use of the pressure detection signal in a manner to be described later.
(2) Then, the drive shaft is driven by the injection motor to be moved forward. A forward end of the screw acts as a piston to inject the molten resin into the mold. This process is referred to as a filling or an injection process. During the injection process, the controller put the screw into a velocity or speed control mode.
(3) At the end of the injection process, the mold is filled with the molten resin. Then, the controller switches a control mode from the velocity control mode into a pressure control mode. Such a switching operation is called a V-P (velocity to pressure) switching operation. The timing and the way of the switching operation seriously affect quality of a molded product.
(4) After the V-P switching operation, the molten resin filled in the mold is cooled and solidified under a predetermined pressure. This process is referred to as a holding process. The pressure applied to the resin is controlled by the controller, like the back pressure.
In the injection unit, the holding process (4) is further succeeded by a next measuring process (1) in a next cycle.
On the other hand, a clamping unit is operated to clamp or close the mold during the holding process (4) which is followed by an ejecting process. In the ejecting process, a molded product which has been cooled and solidified is removed or ejected from the mold by opening the mold and using an ejector mechanism. The mold is thereafter closed again and the ejecting process proceeds to the injection process (2). Thus, the measuring process (1) in the injection unit is carried out simultaneously with the ejecting process in the clamping unit.
In the injection molding machine, it is required to prevent an excessive pressure from being imposed on the mold during the injection process and to suppress a variation of the pressure upon the V-P switching operation.
In the meanwhile, a high-speed injection can not be realized when a load pressure is detected to determine a switching timing on the V-P switching operation due to a time lag which inevitably takes place upon detection of the load pressure. Taking this into account, restriction must be imposed on the injection motor, namely, a drive source during the injection process in some way so as to prevent a filling pressure from exceeding a limit value.
More specifically, when the servo motor is used as the drive source in the injection molding machine, a drive current should be controlled by the controller. In this event, the drive current is often restricted to a predetermined range to control an injection pressure. However, it often happens that the drive current partially exceeds the predetermined range in the velocity or the pressure control mode and that the injection molding machine is put into an uncontrolled state when the drive current exceeds the predetermined range. As a result, control operation is unfavorably affected by disturbance.
At any rate, continuity of an injection pressure can not be kept upon the V-P switching operation. This results in degradation of the quality of the molded product.